Our efforts in the coming year will be directed towards a more precise delineation of the organization of the lymphohemopoietic system. Specifically, we will attempt to isolate highly enriched populations of pluripotent stem cells, and hemopoietic and lymphopoietic progenitors with particular emphasis on the lymphopoietic system. Preliminary experiments using light scatter, fluorescence intensity, velocity sedimentation and density centrifugation suggest that this is a feasible goal. The developmental potential of these enriched cell populations will be studied in vivo and in vitro. In addition we will attempt to trace the development of the 2 lines of thymocytes that we have described in order to determine their relationships (if any) to each other and to subsets of peripheral T cells. Lastly, we will use Thy-1, Ig subclasses and Ia antigens as markers with which to identify and isolate B cell subsets in the rat. The relative contributions of these subsets to primary and secondary antibody responses, immunological memory and immunological tolerance will be of particular interest.